: Nucleus tractus solitarius (NTS) is the brainstem area that receives direct input from visceral sensory afferents. These inputs to NTS second-order neurons initiate reflex regulatory mechanisms important in homeostasis. A key anatomic feature of NTS is reciprocal connections to other brain regions. It is unclear what the significance of such projections might be. One of these regions is the paraventricular hypothalamic nucleus (PVN). PVN contributes to cardiovascular and neuroendocrine responses to physiological or psychological stress. Vasopressin (AVP)-containing PVN neurons play a central role in these responses. However, the cellular mechanisms involved in the hi-directional communication between NTS and other brain regions are unknown. The general hypothesis is that second-order NTS neurons mediate NTS-PVN interactions and that AVP is a key mediator. Our basic approach will be to study second-order NTS neurons in an in vitro medullary slice preparation containing a long section of solitary tract (ST). ST-evoked synaptic currents will be recorded using standard whole cell patch clamp techniques. In preliminary experiments I have recorded from PVN-projecting and second-order baroreceptor NTS neurons identified by fluorescent retrograde tracers. Using this novel combination of tract tracing techniques and cellular electrophysiology three specific hypotheses will be tested: 1) AVP depresses visceral afferent synaptic transmission to second-order NTS neurons, 2) AVP enhances potassium currents in second-order NTS neurons, and 3) A subpopulation of second-order NTS neurons project to PVN .